Making Money and Monstrous regiment are my two least favourite books. I personally don't think Nightwatch is the best, and it doesn't appear in my top five.
After the disappointment of Making Money (and to some degree Wintersmith) I was highly satisfied with Unseen Academicals. I would rate it in the top half of all Discworld books.
Vetinari was wonderful and Professor Hicks I absolutely adored. I am more than happy to swap Hicks for the Bursar (a one joke running gag) AND Dean (a two joke running gag). Nutt was brilliant and I liked the way some of the characters talked in modern 2009 patois.
I used to hate Maurice, but on fourth reading, I fell in love with it.
UA is a very funny book and I believe on your second or third reading of it, you will appreciate it even more.
Incidently, in what ways have the characters of Vetinari, Ponder and Ridcully changed so much, because I didn't notice that much of a change?